Google (as far as I can tell at least) hasn't made any updates to Groups since they did the (really minor) Material Design UI refresh maybe 6+ years ago.
Specifically, there is still no way to manage the "who can send to group" list.
A workaround might be to set it to "anyone on the web can send" but also set it to have all messages moderated. Then, as emails come in from the various "good" senders, you could approve that sender to have all future messages from that sender go through without being held for moderation.
Personally, i think it's pretty clear that they're largely not interested in making further investments in Groups, which is frustrating.....